1. Chemical Engineering Plant Design
Lek Wantha
Lecture 06
Recycle Structure of the Flowsheet and Reactor Design

2. A Hierarchical Approach to Conceptual Process Design
Decide whether the process will be batch or continuous
Identify the input-output structure of the process
Identify and define the recycle structure of the process
Identify and design the general structure of the separation system
Identify and design the heat exchanger network or process energy recovery system

3. Decisions that Determine the Recycle Structure
How many reactors are required?
Should some components be separated between the reactors?
How many recycle streams are required?
Do we want to use an excess of one reactant at the reactor inlet
Is a gas-recycle compressor required?
How does it affect the cost?

4. Decisions that Determine the Recycle Structure
Should the reactor be operated adiabatically, with direct heating or cooling, or is a diluent or heat carrier needed?
Do we want to shift the equilibrium conversion? How?

5. Number of Reactor Systems
If reactions take place at different temperatures and pressures and/or they require different catalysts, then a separate reactor system is required for each operating condition.

6. Number of Reactor Systems
Example 1:
Example 2:

7. Number of Recycle Streams
Depend on process requirement.
Do not separate two components and then remix them at a reactor inlet.
Components recycled to the same reactor that have neighboring boiling points should be recycled in the same stream.
Separate gas and liquid recycle streams.
A gas-recycle compressor is required if the recycled components boil at a temperature lower than that of propylene.

8. Number of Recycle Streams
Example 1: Acetic anhydride

9. Number of Recycle Streams
Example 2: HDA process

10. Number of Recycle Streams
Example 2: HAD process

11. Number of Recycle Streams
Example 3: Acetic anhydride

12. Number of Recycle Streams
Example 3: Acetic anhydride

13. Excess Reactants Shift the product distribution.
Force another component to be close to complete conversion.
Shift the equilibrium conversion.
If an excess reactant is desirable, these is an optimum amount of the excess.

14. Recycle Material Balance Example
See accompanied sheet

15. Reactor Heat Effect Adiabatic Direct heating or cooling (Isothermal)
Addition of diluent or heat carrier

16. Reactor Heat Effect Reactor Heat Load
- Reactor Heat Load = Heat of Reaction x
Fresh Feed Rate
Adiabatic Temperature Change
- QR = FCp(TR,in – TR,out)

17. Reactor Heat Effect

18. Reactor Heat Effect Heat Transfer Area - Area = QR /UdT Heat Carrier
- recycle more reactant or a product or by product
- extraneous component

19. Reactor Heat Effect
For endothermic processes with a heat load of less than 6 to 8 E6 Btu/hr, we use an isothermal reactor with direct heating. For larger heat load we may add a diluent or heat carrier.

20. Reactor Heat Effect
For exothermic processes we use an adiabatic reactor if the adiabatic temperature rise is less than 10 t0 15% of the inlet temperature. If the adiabatic temperature rise exceeds this value, we use direct cooling if the reactor heat load is less than 6 to 8 E6 Btu/hr. Otherwise, we introduce a diluent or a heat carrier.

21. Equilibrium Limitations

22. Equilibrium Limitations

23. Reactor Heat Effect
For single reactions we choose a conversion of 0.96 or 0.98 of the equilibrium conversion.
The most expensive reactant (or the heaviest reactant) is usually the limiting reactant.
If the equilibrium constant of a reversible by-product is small, recycle the reversible by-product.

24. Reactor Design

25. Reactor Design